I would have to say hands are the most difficult thing to draw well. Paintings throughout history have hands hidden behind skirts, baskets, panels, everywhere. Artists know they can't draw them, so they conceal them.
One of the toughest parts of learning to draw is perspective. If your perspective is off, then your drawing isn't going to look great. There are different aspects of perspective to keep in mind. The first is the angle at which the viewer is observing the subject.
Lighting design engineers have been illuminating things with magic glowing powders for at least eighty years. But blue is still a tricky colour with modern LEDs.
One practical reason children are taught to draw before they paint is that drawing requires fewer materials, and when drawing with a pencil, it is easier to erase mistakes as you learn. This learning process implies that drawing is the easier medium that you learn before progressing to painting.
Diamonds remain the most scratch-resistant material known to humanity. Metals like titanium are far less scratch-resistant, and even extremely hard ceramics or tungsten carbide cannot compete with diamonds in terms of hardness or scratch-resistance.
'Of all the arts, abstract painting is the most difficult. It demands that you know how to draw well, that you have a heightened sensitivity for composition and for colours, and that you be a true poet. This last is essential. ' ~ Wassily Kandinsky.
While hands make for stunning drawings, they are notoriously difficult to draw due to the amount of bones, muscle, and tendons in each hand. But don't let that intimidate you. Simplify the process and break hand drawing down into basic shapes and manageable steps, and you'll be on your way to drawing a lifelike hand.
Hard skills are measurable technical skills, which are the ones students primarily focus on. These skills include drawing from life, drawing from reference, challenging yourself, not overcomplicating your work.
General rule is that male is easier to draw than a female in humans. The reason is a man has a rougher skin which is a bit easier to render than a smooth skin which a woman has.
Tooth enamel is the first line of defense your teeth have against plaque and cavities. It is the white, visible part of the tooth and it is also the hardest part of the human body.
Some of the muscles that grip your pen attach to the bones of your fingers and run up the forearm to the elbow. Tension in the fingers can contribute to repetitive stress injuries in the hand, wrist, forearm, and elbow.
It's possible to see improvements by drawing only 1-2 hours per day. But if you want to see significant improvements you should be aiming for 5-6 hours per day, or more if possible. Starting anywhere is better than never starting.
Sketching and drawing is a great way to improve your creative skills and start thinking in a different way. Art shows you that there is normally more than one way to solve a problem. Art encourages open ended thinking and creativity. These can be really helpful for personal development and solving problems.